Enhanced microstructure and mechanical properties of ZrN-reinforced AlSi10Mg aluminum matrix composite

Aluminum matrix composites (AMCs), incorporating Zirconium Nitride (ZrN) as reinforcing additives, demonstrate immense promise for applications in aerospace, automotive, and power generation due to their unique combination of low density, superior mechanical properties, and excellent thermal/electrical conductivity. This study explores the influence of ZrN reinforcement on the microstructure and mechanical properties of AlSi10Mg metal-matrix composites. Utilizing high-energy ball milling (HEBM) and spark-plasma sintering (SPS), ZrN/AlSi10Mg composites were synthesized, achieving nearly full density with uniform ZrN distribution, while phase and chemical transformations were not observed in the bulk composites. The addition of ZrN resulted in a notable increase in hardness of 237% (182 ± 8 HV2), elastic modulus of 56% (114 ± 3 GPa), compressive and tensile strength of 183% (565 ± 15 GPa), and 125% (387 ± 9 GPa), respectively, for composites containing 30% ZrN, compared to the non-reinforced alloy. Experimentally determined coefficients of thermal expansion (CTEs) for composites with 10%, 20%, and 30% ZrN content were 19.8 × 10−6 °C−1, 19.1 × 10−6 °C−1, and 18 × 10−6 °C−1, respectively, which well relates to Schapery’s model. These findings contribute to understanding the synthesis, mechanical behavior, and thermal properties of ZrN/AlSi10Mg composites, demonstrating their potential for diverse engineering applications.


Feedstock powders
AlSi10Mg powder of spherical morphology (Fig. S1a) with an average particle size of ~ 42 μm (Fig. S1b) was used as a matrix material, and its chemical composition is given in Table S1.The reinforcing ceramic component was ZrN obtained by combustion synthesis 31,32 .ZrN particles have a fragmented morphology (Fig. S1c) with an average size of ~ 2 μm (Fig. S1d).The diffraction patterns of the initial powders are shown in Fig. S2.As can be seen, AlSi10Mg consists of two phases: α-Al Fm-3m (225) and Si Fd-3m (227).Meanwhile, the diffraction pattern of ZrN contains peaks corresponding only to a cubic phase Fm-3m (225), which indicates that the synthesized powder is single-phase.

Fabrication of bulk ZrN/AlSi10Mg composites
In this study, the ZrN/AlSi10Mg compact composites were produced by the powder metallurgy method.high-energy ball milling (HEBM) in an Activator-2S planetary ball mill (Chemical Engineering Plant Ltd., Russia).The HEBM of the mixture was carried out under an argon atmosphere (0.6 MPa) in 250 ml steel jars with 6-mm steel balls as grinding media at a 15:1 ball-to-powder mass ratio (300g:20g).The rotation speeds of the planetary disk and jars were equal and amounted to 650 rpm, HEBM duration of 10 min.To prevent the sticking of powders to the grinding bodies and the walls of the pots, stearin was added.
In order to obtain ZrN/AlSi10Mg compacts, the prepared AlSi10Mg with 10, 20, or 30 wt% ZrN powder mixtures were sintered in a Labox 650 spark plasma sintering unit (SinterLand, Japan) in vacuum under a load of 50 MPa at 500 °C for 20 min.Heating was carried out at a constant rate of 100 °C/min.Initial AlSi10Mg powder with no additives was sintered in the same mode without preliminary HEBM.Through this process, disks with a diameter of 30 mm and a thickness of 7-8 mm were fabricated.

Characterization and properties
The phase composition of the bulk aluminum-matrix composites ZrN/AlSi10Mg was studied using a DRON X-ray diffractometer (Burevestnik, Russia) with Cu-Kα radiation in the step-by-step scanning mode (shooting step is 0.1°) with an exposure of 2 s.Phases were identified using PDF 2 (2004) database.
The microstructures of the powders and bulk samples after HEBM and SPS were investigated via a JEOL JSM7600F scanning electron microscope (JEOL Ltd., Japan) equipped with an X-MAX 80 mm 2 attachment (Oxford Instruments, UK) for X-ray microanalysis.Images were obtained at an accelerating voltage of 15 kV.The fine structure was studied by high-resolution transmission electron microscopy (TEM) on a JEM-2100 microscope (Jeol, Japan).The High-Resolution Cross-sectional Transmission Electron Microscopy (x-HRTEM) data were obtained from the above lamella with an electron microscope JEM-2100 Plus (JEOL, Tokyo, Japan) operating at 200 kV.The electron beam current in the scanning transmission electron microscopy (STEM) mode was 1 nA.The lamella was treated with a focused ion beam at an accelerating voltage of 30 kV.During the cross section, the current was 0.79 nA, and during polishing, it was 24 pA.Particle size analysis was performed on a Bettersizer ST analyzer (Bettersize Instruments LTD, China) with wet dispersion in distilled water.The flow rates of the powders of a weighted quantity of 50 g were assessed using a Hall flowmeter installation (ASTM B213-17).
Relative density (ρ) was determined as the ratio of pycnometric density (ρ p ) measured using an Ultrapyc-nometer1000 helium pycnometer (Quantachrome Instruments, USA) and theoretical density calculated by rule of mixture.
Vickers hardness was measured using a Durascan-70 digital hardness tester (Struers ApS, Denmark) at a maximum load of 20 N and an exposure time of 10 s.The measurement of Young's modulus and nanohardness was conducted on a Micro-Hardness Tester (CSM Instruments, Switzerland) under an applied load of 50 mN and at an exposure time of 10 s.The Oliver-Pharr technique was used to interpret the collected data 33 .
Tensile testing was carried out on samples with a rectangular cross-section of 2 × 6 mm 2 and a length of 15 mm using an Instron 5966 universal testing machine (Instron, USA).Compress testing was conducted for cylindrical samples with a height-to-diameter ratio of 2:1 on a hydraulic testing machine PGM-100MG4 (SKB StroyPribor LLC, Russia).The loading rate in tension and compression was 5 and 0.6 MPa/s, respectively.The CTE was studied on cylindrical samples with a diameter of 4 mm and a height of 10 mm on a quenching dilatometer DIL805A/D (TA Instruments, Germany) at a heating rate of 5 °C/min.At least five hardness and tensile/ compress strength measurements were taken for each sample.

Characterization of powder mixtures subjected to HEBM
In order to form composite particles and, therefore, evenly distribute ZrN in the AlSi10Mg matrix, powder mixtures with 10, 20, and 30 wt% ZrN were subjected to HEBM.The SEM images and particle size distribution curves of resulting powders are demonstrated in Fig. 2. Introduction of 10 wt% ZrN has led to noticeable changes in morphology and has caused a decrease in particle size compared to the original AlSi10Mg powder.In addition to plate-like particles, round-like particles (Fig. 2a) are observed in the composite powders 10Z, which formation, we suppose, has been caused by hardening during the HEBM process.On the other hand, composite powders 20Z (Fig. 2b) consist predominantly of round-like particles; however, plate-like particles can be also observed occasionally.The particles of 30Z powder (Fig. 2c) are more spherical than others containing lower ZrN fraction.
According to the size distribution data, the particle size for 10Z composite varies from 0.5 to 120 μm, and De Brouckère diameter (D [4,3]) equals ~ 36 µm (Fig. 2d).As both 20Z and 30Z compositions are characterized by close D [4,3] values of ~ 35 and ~ 31 µm, respectively (Fig. 2e,f), which is slightly lower compared to the powder 10Z.Despite that the distribution curves of all samples are similar and D [4,3] values are almost identical, the flow rate of the powders significantly decreases with decreasing ZrN content, which is probably connected to morphological differences.The 10Z и 20Z powders do not show flowability, on the contrary, the flow rate of 30Z is found to be 18 s.
XRD patterns of the powder mixtures subjected to HEBM are presented in Fig. S3.In the case of all powder mixtures, there are three well-crystalline phases: α-Al Fm-3m (225), Si Fd-3m (227), and ZrN Fm-3m (225).The lattice parameters of Al, Si, and ZrN are found to be 0.4045, 0.5436, and 0.4567 nm, respectively.The parameters remain constant for all mixtures and are comparable with the feedstock powder, which confirms the absence of chemical interaction between the components during the HEBM process.For mixtures with 10, 20, and 30 wt%, the amount of ZrN phase calculated by the Rietveld method was 9, 19.2, and 29.6 wt%, respectively.Therefore, ZrN losses resulting from powder adhesion to milling media and pot walls during HEBM do not exceed 1 wt% and are considered insignificant.
The particles of a ductile component (AlSi10Mg) plastically deform upon collision with grinding media, which leads to flattening the particles, while the particles of a fragile component (ZrN) are split into smaller ones.As it is known, there is a critical deformation degree upon which cold welding takes place 34 .One should expect that during the HEBM local deformation near ZrN particles increases, which improves the cold-welding process, as it is observed in 6061 aluminum alloy with AlN 35 .This may explain why particles are flattened at a low additive content of 5 wt% of ZrN (Fig. S4), showing similar behavior to other ductile metals where plastic deformation prevails [36][37][38] .An increase in the number of fragile and hard particles increases the number of areas where severe plastic deformation occurs that, in turn, facilitates the grinding and cold welding of AlSi10Mg.Accordingly, the process of cold welding begins earlier than in systems with lower ZrN concentrations (Fig. S5).Upon this process, grounded ZrN particles are disposed between flattened AlSi10Mg ones, i.e. they are located on the boundaries of AlSi10Mg.Repeatable plastic deformation, grinding, and cold welding cause the formation of round-shaped composite particles.The cross sections of 10Z sample show that there are two particle types: (i) composite particles with ZrN homogeneously distributed in AlSi10Mg matrix; (ii) decorated particles where ZrN is located along the surface of deformed AlSi10Mg particles.Other particles without ZrN shown in Fig. 2g-i are silicon oxide, which is a component of a resin used for metallographic sample preparation (Fig. S6).
Subsequent increasing the mass fraction of ZrN causes more spherical particles to appear.As a result, the composite particles of 30Z sample are more spherical than others, which is responsible for its flowability.In the case of the powder mixtures 20Z and 30Z (Fig. 2h,i, Figs.S7, S8), only composite particles were found.One more explanation for the variation in powder morphologies with low (Fig. S4) and high (Fig. 2c,i) ZrN content is that these ceramic particles can serve as additional milling media 35,37,39 .
Overall, high-energy HEBM processing (650 rpm) in a planetary ball mill for 10 min promotes the formation of ZrN/AlSi10Mg composite particles.Zirconium nitride serves as the milling medium, significantly reducing the duration of plastic deformation of the metal powder and activating the fracture mechanism.This results in the formation of composite particles with a rounded shape within the size range of approximately 31-36 µm and a relatively uniform distribution of reinforcing phase in the metal matrix.www.nature.com/scientificreports/

Spark plasma sintering
The main morphological and chemical transformations in the composites, affecting their functional properties occur during the SPS process.To track the changes and study features of morphology and phase composition of the obtained composites XRD and SEM/EDX analysis were performed.Figure 3 shows the XRD profiles of ZrN/AlSi10Mg aluminum-matrix composites with different weight fractions of ZrN after SPS.The peak positions of the α-Al Fm-3m (225), Si Fd-3m (227) and ZrN Fm-3m (225) phases after SPS remain unchanged compared to the powder mixtures after HEBM, which indicates an absence of chemical interaction between components.
The microstructures of sintered AlSi10Mg and composites 10Z, 20Z, 30Z, as well as element distribution maps are shown in Fig. 4. The microstructure of the pure AlSi10Mg (Fig. 4a) has a subtle compositional contrast corresponding to the elements Al and Si, which is in good agreement with the diffraction pattern of AlSi10Mg after SPS (Fig. 3).The microstructures of the ZrN/AlSi10Mg composites (Fig. 4b-e) are characterized by the presence of two types of phases: gray matrix and white inclusion.The analysis of the EDS results (Fig. 4f-i) has revealed that the gray-colored areas correspond to Al and Si, while the white-colored areas-to the ZrN reinforcing particles.www.nature.com/scientificreports/TEM was additionally utilized to accurately examine the phase composition of the 30Z sample (Fig. 5).Three constituent phases were identified in the bright-field mode (Fig. 5a).EDX (Fig. 5b-e) and SAED (Fig. 5f-h) results have revealed that the structure of 30Z composite consists of the matrix containing Al and Si with embedded ZrN particles.The results obtained demonstrate the absence of interaction between ZrN and Al-matrix, and the formation of intermetallics on the phase boundary has not been observed (Fig. 5i).
The density values of the sintered samples determined using the helium pycnometer are 2.6727 ± 0.0005, 3.0833 ± 0.0007, 3.5032 ± 0.0007, 3.9709 ± 0.0003 g/cm 3 for AlSi10Mg with no additives and with 10, 20, 30 wt% ZrN, respectively.The theoretical densities of the composites were calculated using the rule of mixtures (ρ AlSi10Mg = 2.68 g/cm 3 ; ρ ZrN = 7.09 g/cm 3 ) and were found to be 3.1144, 3.5208 and 4.003 g/cm 3 for the 10Z, 20Z, and 30Z, respectively.Therefore, it can be concluded that the relative density of the sintered samples is ≥ 99%.
Overall, the SPS-obtained ZrN/AlSi10Mg composites show no phase or chemical transformation during the sintering process but exhibit significant densification, likely as a result of solid-state mass transfer.The sintering process allows to obtain composites with a nearly fully dense structure.

Mechanical properties
Mechanical properties (hardness, elastic modulus, compressive, and tensile strength) were measured on the sintered AlSi10Mg and ZrN/AlSi10Mg samples, and the results are shown in Fig. 6.
Figure 6a shows the HV 2 hardness and elastic modulus depending on the ZrN fraction.The average hardness of the AlSi10Mg aluminum alloy after SPS is 54 ± 5 HV 2 .The hardness of the composites is substantially higher and gradually increases from 115 ± 6 HV 2 (10Z) to 182 ± 8 HV 2 (30Z) with the increasing mass fraction of ZrN, which is respectively 112% and 237% higher compared to pure AlSi10Mg.The indentation test has shown that the elastic modulus changes similarly demonstrating a maximum at 114 ± 3 GPa (30Z), which is 56% greater than that of pure AlSi10Mg (73 ± 2 GPa).
The ultimate compressive strength (UCS, Fig. 6b) was measured on cylindrical specimens with a height-todiameter ratio of 2:1.As can be seen, UCS monotonically increases with the increasing mass fraction of ZrN.The highest UCS value of 565 MPa is observed for the sample 30Z, which is 183% higher compared to pure AlSi10Mg, for which the UCS value is 200 MPa (the value was recorded before necking occurred).Figure 6b shows the appearance of the samples after compression tests.An applied load to the AlSi10Mg sample has caused plastic deformation leading to a change in the cross-sectional area and a displacement of the upper part of the sample relative to the axis along which the load was applied.All the ZrN/AlSi10Mg composites failed by shear banding oriented about 45° with respect to the loading direction.
Figure 6c shows the tensile curves of the AlSi10Mg and ZrN/AlSi10Mg composites.Pure AlSi10Mg deforms significantly, and the elongation is 25%, but a yield plateau has not been observed.On the other hand, the ZrN/ AlSi10Mg composites do not show ductility, and the fracture is observed in the elastic region.The introduction of the ZrN ceramic additive has a positive effect on the strength of AlSi10Mg.In the range of 10-30 wt% ZrN tensile strength increases monotonically, reaching a maximum value of 387 ± 9 MPa for the 30Z, which is 125% higher compared to the pure AlSi10Mg.Figure 6d shows the values of Vicker's hardness and ultimate tensile strength of the ZrN/AlSi10Mg (cubes) in comparison with AMCs obtained by various compaction methods.
The fracture surfaces of the AlSi10Mg and ZrN/AlSi10Mg samples with different ZrN contents after tensile tests are demonstrated in Fig. 7.The fracture surface of the AlSi10Mg (Fig. 7a) appears to be rough having dimples of different sizes with sharp ridges, which indicates ductile fracture of the material and is in good agreement with the stress-strain curve (Fig. 6c).An increase in the mass fraction of ZrN to 10 wt% (Fig. 7b) leads to a noticeable change in the morphology of the fracture surfaces.The number of dimples has noticeably decreased in comparison with pure AlSi10Mg, and ridges are found in some places on the fracture surface.Additionally, exposed ZrN and cracks are observed.As the mass fraction of ZrN increases to 20 and 30 wt% (Fig. 7c-f), the amount of exposed ZrN increases.At higher magnification, it can be seen (Fig. 7e,f) that some exposed ZrN particles are located in the cavities and are separated from the cavity walls, while the rest are firmly fixed in the matrix.Some exposed ZrN particles show signs of chipping.From SEM images of the fracture surface of the ZrN/AlSi10Mg composites, one can conclude that the fracture is of the mixed nature, in which the brittle fracture mechanism (chips on ZrN particles) prevails over the ductile one (AlSi10Mg dimples and ridges).www.nature.com/scientificreports/promotes the formation of micro-cracks in ZrN particles.The stress concentration at the tips of these defects that arises under the influence of the load leads to the destruction of ZrN particles by chipping.As a result, the composites are destroyed (Fig. 8c).A similar failure mechanism has been observed in other AMCs with ceramic reinforcements [40][41][42][43] .
Mechanical properties significantly depend on the fabrication technology and matrix materials used.Therefore, it is more correct to evaluate the effectiveness of reinforcing additives compared with the original metal or alloy with no additives.In this work, the addition of 10-30 wt% ZrN to the AlSi10Mg alloy has increased hardness, compressive, and tensile strength by 112-237%, 87-183%, and 48-125%, respectively.The high mechanical properties of the ZrN/AlSi10Mg composites may be due to both strain hardening during the HEBM process and the reinforcement effect.As previously said, during the HEBM process, the aluminum matrix undergoes plastic deformation resulting in decreasing grain size eventually leading to strain hardening of the composites 53,54 .Moreover, reinforcement may also contribute to an increase in mechanical properties due to the following: (1) transferring the applied load from the plastic AlSi10Mg matrix to solid ZrN particles; (2) grain refinement due to the addition of ceramic ZrN particles resulting in enhancing properties according to the Hall-Petch relation; (3) elastic stress appearing because of a severe difference between the CTE of AlSi10Mg (20.5 × 10 −6 °C−1 ) and ZrN (7.24 × 10 −6 °C−123 ).
Overall, the results highlight that the synergistic effects of deformation strengthening during HEBM and reinforcement lead to a significant enhancement in the mechanical properties of ZrN/AlSi10Mg.This improvement becomes more pronounced with an increasing mass fraction of ZrN.However, during the tensile testing of the composites, produced via HEBM followed by SPS, a mixed fracture mode is evident, where the brittle fracture mechanism prevails over the ductile one.The CTE values obtained experimentally have the greatest agreement with the Schapery model (Fig. 9a).CTE of the composites 10Z and 20Z lies on the dotted line corresponding to Schapery's upper limit.At the same time, the CTE of the composite 30Z is located slightly below Schapery's upper limit.The CTE values calculated by the Turner model and ROM do not match the experimental data (Table 2) and do not allow the prediction of CTE with sufficient accuracy.The good convergence of the experimental CTEs with Schapery's model may indicate a good interconnection between the components of the AlSi10Mg and ZrN composite 58 , as well as the presence of a complex mechanical interaction between them.Therefore, Schapery's model, which considers shear and volumetric stresses, is the most suitable for predicting the CTE of ZrN/AlSi10Mg.

Conclusion
In this study, bulk ZrN/AlSi10Mg composites with 10, 20, and 30 wt% ZrN were successfully synthesized.Utilizing suitable HEBM processing parameters, bulk samples with excellent mechanical strengths and thermal properties were fabricated via SPS.The obtained results enable us to conclude the following:    www.nature.com/scientificreports/ (1) With increasing ZrN fraction from 10 to 30 wt%, parameters D50 and D [4,3], corresponding to mean particle size, decrease.At the ZrN content of 10 wt%, ductile AlSi10Mg particles undergo plastic deformation leading to both flattened and spherical morphologies with the formation of composite and roundlike particles.As ZrN content increases from 20 to 30 wt%, fragmentation and flattening effects cause the formation of mainly composite round-like particles, while 30 wt% of ZrN provides flowability suitable for additive manufacturing.(2) XRD, SEM, and TEM observations have revealed that bulk ZrN/AlSi10Mg composites sintered by SPS at 500 °C do not demonstrate any phase and chemical transformations under sintering.The cell parameters and peak positions of main phases remain unchanged upon SPS compared to the powder mixtures after HEBM.(3) The incorporation of ZrN particles significantly enhanced the mechanical properties of bulk ZrN/AlSi10Mg composites, resulting in a remarkable increase in hardness, elastic modulus, compressive strength, and tensile strength.These mechanical properties exhibited a consistent improvement with an increase in ZrN content from 10 to 30 wt.For a composite with 30 wt% ZrN, the hardness, elastic modulus, compressive strength, and tensile strength values were 182 ± 8 HV2, 114 ± 3 GPa, 565 ± 15 GPa, and 387 ± 9 GPa, respectively.These values represent a substantial improvement of 237%, 56%, 183%, and 125% compared to the non-strengthened AlSi10Mg alloy.The results contribute to the understanding of the synthesis, mechanical behavior, and thermal properties of novel bulk ZrN/AlSi10Mg composites.By changing the amount of additive, the target parameters of the composite can be adjusted: cost, relative density, mechanical, and thermophysical properties.Each compound developed has potential for advanced engineering applications, especially round-like composites, which may be suitable for additive manufacturing.

Figure 9a shows the
Figure9ashows the dependence of the experimentally measured CTE of ZrN/AlSi10Mg on the volume fraction of ZrN, as well as the calculated CTE by the ROM (rule of mixture) models55 , Turner56 , and Schapery57 .The experimentally measured CTE of the AlSi10Mg and ZrN/AlSi10Mg composites are indicated by solid squares, and the results of theoretical calculations are presented as lines: continuous (ROM), dash double dot(Turner), dashed (Schapery upper bound), dash-dot (Schapery lower bound).Detailed descriptions of the theoretical calculations of the CTE are presented in the Supplementary file.The CTE values obtained experimentally have the greatest agreement with the Schapery model (Fig.9a).CTE of the composites 10Z and 20Z lies on the dotted line corresponding to Schapery's upper limit.At the same time, the CTE of the composite 30Z is located slightly below Schapery's upper limit.The CTE values calculated by the Turner model and ROM do not match the experimental data (Table2) and do not allow the prediction of CTE with sufficient accuracy.The good convergence of the experimental CTEs with Schapery's model may indicate a good interconnection between the components of the AlSi10Mg and ZrN composite58 , as well as the presence of a complex mechanical interaction between them.Therefore, Schapery's model, which considers shear and volumetric stresses, is the most suitable for predicting the CTE of ZrN/AlSi10Mg.For the 10Z, 20Z and 30Z CTE are 19.8 × 10 −6 °C−1 , 19.1 × 10 −6 °C−1 , and 18 × 10 −6 °C−1 , respectively (Table2), which is by ~ 3, 7, and 12% lower than for pure AlSi10Mg (20.5 × 10 −6 °C−1 ).As can be seen from Fig.9band Table2, the ZrN/AlSi10Mg have CTE values comparable to other AMCs[59][60][61][62][63] .For example, the ZrN/ AlSi10Mg with 30 wt% ZrN exhibits lower CTE compared to Al-12 vol% B 4 C 61 , AlSi10Mg-SiC-TiB 262 , and 10% Al 2 O 3 /606163 .However, the CTE of composites containing SiC 60 is slightly lower than that of the ZrN/AlSi10Mg.

Figure 9 .
Figure 9. CTE values of the ZrN/AlSi10Mg composites.(a) Experimentally measured CTE of the AlSi10Mg and ZrN/AlSi10Mg with different ZrN content, as well as calculated CTE in accordance with the ROM and Schapery models; (b) CTE of the ZrN/AlSi10Mg obtained and other AMCs.

( 4 )
Under tension, the ZrN/AlSi10Mg composites exhibit a complex fracture mechanism.The mechanism involves plastic deformation in the matrix, which leads to neck formation.Partial decohesion at the AlSi10Mg-ZrN boundary causes micro-voids, and subsequent deformation results in the tearing of necks, leaving ridges on the fracture surface.The cracks propagate along the boundary, transferring the load to ZrN particles bonded to the matrix.Moreover, HEBM contributes to micro-crack formation in ZrN particles, ultimately leading to composite destruction.(5)The experimentally determined coefficients of thermal expansion (CTEs) for theZrN/AlSi10Mg composites align well with Schapery's model, considering both volumetric and shear stresses.The CTE values for ZrN/AlSi10Mg composites with 10%, 20%, and 30% ZrN content are 19.8 × 10 −6 °C−1 , 19.1 × 10 −6 °C−1 , and 18 × 10 −6 °C− 1, respectively.